The present invention relates broadly to the precise registration and alignment of two or more planar workpieces with respect to each other. In particular, the invention relates to a control board used to support a photosensitive lithographic plate and one or more duping film overlays relative to a light exposure assembly, whereby images are formed on the plate, and wherein proper registration of the plate to the overlays is critical to image clarity and definition.
Each exposure is performed in a vacuum frame. The lithographic plate and film overlays are provided with registration holes which are punched at identical spacings and hole configurations to ensure uniformity. The lithographic plate and films are placed on the image control board at registration pins which align the plate and films to one another and the vacuum frame. A glass plate of the vacuum frame covers the lithographic plate and films. A rubber diaphragm in the vacuum frame compresses the film, lithographic plate, and image control board against the glass plate.
In the prior art, the contacting of the film with the plate often times caused the registration pin to distort the film around the pin hole with consequent buckling of the film and loss of registration. For multi-color printing applications, proper alignment and registration versus misalignment is commonly determined by registration to a few thousandths of an inch. Thus, any buckling or non-conformity between the plate and films can result in unusable plates or press runs.
An early attempt to prevent the loss of registration was the use of discreet spring loaded, retractable pins. These pins were loosely positioned on the support cushion beneath the plate and film overlays and were subject to canting.
Also known in the prior art are image control boards that are formed of a rigid material, for example, acrylic plastic sheets. Multiple registration pins are secured to these boards in fixed mountings and are positioned to tolerances of a few thousandths of an inch. However, the differing thermal properties of the components of these boards which, for example, are made of steel, stainless steel, brass, aluminum and acrylic plastic, can effect pin alignment to varying degrees.
A solution to the latter problem is proposed in U.S. Pat. No. 4,977,683. There registration pin assemblies are provided that can be manipulated in a laminated board in a multidirectional fashion within the plane of the board to maintain precise and repetitive registration of the lithographic plate and film overlays, while maintaining planarity with the glass plate. The projecting pins are also rotatively mounted within a barrel shaped support of each assembly.
A problem which has been encountered with the latter pins is that the tolerances of the pins and their mountings to the image control board can require extraneous interventions by the vacuum frame operator, before proper registration is achieved. In particular, one or more of the pins can exhibit a preferential, positional alignment, such that each pin must be properly rotated to a particular position before registration is achieved between all pins, the plate and film overlays. Repeated accessing and repositioning of the films, in turn, can waste time and can result in damage to the film overlays.